1. Field of the Invention
The present invention generally concerns nuclear magnetic resonance tomography (MRT) as used in medicine for examination of patients. The present invention concerns a method, as well as an MRT system for implementation of the method, that significantly increase the effectiveness of slice-selective multi-slice excitation in MRT imaging.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully used for over 15 years as an imaging method in medicine and in biophysics. In this examination modality the subject is exposed to a strong, constant magnetic field. The nuclear spins of the atoms in the subject, which were previously randomly oriented, are thereby aligned.
Radio-frequency fields can now excite these “ordered” nuclear spins to a specific oscillation. In MRT, this oscillation generates the actual measurement signal which is acquired by means of suitable reception coils. By the use of inhomogeneous magnetic fields generated by gradient coils, the measurement subject can be spatially coded in all three spatial directions. The method allows a free selection of the slice to be imaged, whereby slice images of the human body can be acquired in all directions. MRT as a slice image method in medical diagnostics is distinguished predominantly as a “non-invasive” examination method with a versatile contrast possibility. Due to the excellent capability of representing soft tissue, MRT has developed into a method superior in many ways to x-ray computed tomography (CT). MRT today is based on the application of spin echo and gradient echo sequences that enable an excellent image quality with measurement times on the order of minutes.
To examine larger segments of a patient or, respectively, for whole-body acquisitions, a continuous table displacement (Move During Scan, MDS) or a step-by-step table displacement (in the z-direction, i.e. in the direction of the patient longitudinal axis) can advantageously be combined with a three-dimensional slice-selective multi-slice excitation. However, the quality of a slice-selective 3D imaging is strongly dependent on the profile of the respectively employed RF excitation pulse. This profile is not ideally rectangular (i.e. perpendicular edges defining an exactly horizontal amplitude therebetween) but rather normally exhibits more or less angled edges on both sides while the amplitude deviates from a linear course. Such a real profile is contrasted with an ideal profile in FIGS. 2a and 2b. The response or signal function (response function) of the system to such a non-ideal, non-rectangular excitation profile is likewise not ideal and appears as image inhomogeneities in the slice coding direction (z-direction).
According to the prior art, one possibility to prevent such image artifacts in the z-direction is to demarcate the FOV of the respective excitation block (slab) exclusively from the most substantially horizontal region of the respective non-ideal RF excitation pulse. A problem resulting therefrom is a fold-over of signal portions into the FOV (in the excitation block) given non-sampling of the RF pulse edges; namely signal portions that are generated by non-sampling of the edges of the non-ideal RF pulse that are situated outside of the FOV.
In particular given a step-by-step scanning in the z-direction in which the image data sets acquired block-by-block are likewise added to one another in the z-direction, this problem leads to a permeation of the entire region to be imaged with foldovers, which ultimately leads to an extremely poor image quality.
According to the prior art this problem is addressed by causing, in spite of the limitation of the FOV or FOVs horizontal width, the spatial coding (i.e. the k-space scanning) to occur along the entire RF pulse width of every RF excitation pulse, meaning that all edges are taken into account or will be in the coding and ultimately in the later image reconstruction. What is known as an oversampling of the edge regions thus ensues, which ultimately leads to a corresponding measurement time extension of an undesirable duration
In order to keep the duration of the examination (data acquisition) within largely acceptable limits, the sharpness of the RF excitation pulse (the slab profile) must be optimized (meaning that its edges are made steeper), which in turn requires an increase of the RF pulse excitation energy and (to the disadvantage of the patient) increases the energy exposure (the specific absorption rate, SAR) in the tissue to be examined to values that are in many cases unacceptable.
As the preceding discussion shows, in MRT, according to the present prior art, the above-described problem is addressed by a compromise solution. A loss of the scan efficiency by oversampling is accepted, but only to a certain degree by the extent of the slab profile being limited, only as long as operation within the SAR limits can be achieved.